President Obama seems on the verge of the most abject diplomatic capitulation in American history — to Iran, our bitterest enemy — and Republicans are arguing about Donald Trump? The prospect of a deal with Iran is dumbfounding and infuriating, as the U.S. held all the cards in the protracted negotiations and yet executed serial surrenders to the Iranians, rather like a courtier bowing his way backward from a monarch.

While the Obama administration is paving the way for a possible mushroom cloud over Tel Aviv or New York in the near term, we're all tying ourselves in knots about what Trump said about Mexicans. The Democrats seem to have a Trump card.

Immigration arouses tremendous rage among both left and right. The left, always panting to push grievance buttons, transforms illegal immigrants into yet another clientele — as if those who enter the country illegally are entitled to legal status, benefits and even citizenship. They establish "sanctuary cities" as if enforcing immigration laws amounts to persecution.

This drives the right crazy. You don't break into my house and then demand the keys, they fume. While I like a good brawl as much as the next person, it seems that Trump is the answer only if the question is: Why can't we get more oafish egomaniacs into politics? Just when the Republican Party needs finesse and sensitivity when discussing immigration, just when it needs to focus on issues that unite all sectors of the electorate including Hispanic and Asian voters, it gets a blowhard with all the nuance of a grenade.

Trump's smear about Mexican immigrants was about as far away as you can get from Ronald Reagan's, "Hispanics are Republicans; they just don't know it yet." Trump tarred most Mexican immigrants as drug-dealers, criminals and rapists, allowing only as an afterthought that some may be good people. He claimed to have discussed the matter with border guards. (Would those officers please step forward?) In any case, crude and vulgar people always preen that they are brave truth tellers.

Trump has achieved his objective — making himself the center of attention — but he has subtracted from our sum total of knowledge about the immigration issue. According to an analysis of Census Bureau data by the Immigration Policy Center, only 1.6 percent of immigrant males between the ages of 18-39 are incarcerated, compared with 3.3 percent of the native-born. There are terrible stories of immigrants committing crimes, and it's certainly fair to demand that criminal aliens be deported with dispatch. Sanctuary cities are a disgrace. But just as Dylann Roof doesn't represent white people, Mexican rapists don't represent anyone other than themselves.

Oddly, the entire brouhaha over immigration may be misplaced. If demographers are correct, the great wave of immigration from Latin America is over. As Jonathan V. Last noted in the Los Angeles Times, birthrates are plunging throughout our hemisphere. Between 1970 and 2005, Mexico was the source for roughly two-thirds of the million or so immigrants who entered the United States yearly. When this huge migration began, Mexico's birthrate was 6.72 children per woman. It has since fallen to 2.1, and it continues to decline. Last writes: "Countries with fertility rates below the replacement level tend to attract immigrants, not send them." And voila, since 2005, net migration from Mexico has been zero.

The Census Bureau reports that starting in 2013, the country that sent the most immigrants to the U.S. was China, with 147,000, followed by India, with 129,000, and Mexico, with 125,000 (an equal number of Mexicans living here went home). Other Latin American nations whose fertility rates have already dropped below replacement — Brazil, Chile, Uruguay and Costa Rica — send virtually no immigrants north.

Some immigrants will doubtless continue to arrive. As Tamar Jacoby of ImmigrationWorks USA put it, "Once they see that you don't have to bribe the police here, they're satisfied." There remains much to recommend the USA as a destination, and we've been lucky that our neighbors to the south roughly share our religion and civilization (unlike the Muslim immigrants who've flooded Europe during the same period). But with world fertility rates declining, the U.S. may face an unexpected problem: too few immigrants. In 1960, half of the U.S. workforce consisted of high school dropouts. Today, it's only 6 percent. Yet the jobs for low-skilled workers — busboys, chambermaids, food processing — remain. If employers cannot find workers for those jobs, there will be fewer managerial and executive positions for native-born Americans.

It's a complex subject that deserves grownup discussion — exactly what Trump and his claque preclude.